There is a growing tendency to include lipid products containing polyunsaturated fatty acids derived from various fermentation processes in foodstuffs. This is of importance in the recently established desirability to incorporate certain polyunsaturated fatty acids in an infant formula.
Various processes have been described for the fermentative production of lipids or oils containing polyunsaturated fatty acids. Examples are EP-A-155,420 for the production of .gamma.-linolenic acid (GLA)-containing lipid from Mortierella, EP-A-223,960, EP-A-276,541 and WO-A-92/13086 for the production of arachidonic acid (ARA)-containing oil from Mortierella and/or Pythium, WO-A-91/07498 and WO-A-91/11918 for the production of docosahexaenoic acid (DHA)-containing oil from Crypthecodinium cohnii or Thraustochytrium, and WO-A-91/14427 for the production of eicosapentaenoic acid (EPA)-containing oil from Nitzschia. Typically, the microbial species producing the lipid containing the desired polyunsaturated fatty acid(s) is cultured in a suitable medium and the biomass is harvested before the desired lipid obtained.
To obtain a lipid concentrate which has a relatively high triglyceride content typically a nonpolar solvent for the lipid (e.g. hexane) or supercritical CO.sub.2 is used in the extraction process. For example, EP-A-246,324 describes a fractional extraction process for the isolation of lipids from Mortierella, to obtain different extracts which are enriched in either polar or nonpolar (neutral) lipids. The neutral lipid extract still has, however, a relatively low triglyceride content (89.3%) and a high sterol content (9.4%). U.S. Pat. No. 4,857,329 describes an extraction process comprising the use of supercritical CO.sub.2 to selectively elute neutral lipids from Mortierella biomass. However, the triglyceride content of the lipid extract does not exceed 86%.
Yamada et al, Industrial applications of single cell oils, Eds. Kyle and Ratledge, 118-138 (1992) describe an arachidonic acid-containing oil extracted from Mortierella alpina biomass using hexane. The purified oil has a triglyceride content of 90%.
Thus, until now it has not been possible to obtain a microbial triglyceride oil with a high triglyceride content, i.e. 95% or higher, using previous fermentation and extraction technology. It has also not been possible to prepare oils having a particularly low (e.g. less than 1.5%) or high (e.g. at least 10%) sterol content.